PV devices convert photo-radiation into electrical current. Generally, a thin film PV device includes two conductive electrodes sandwiching a series of semiconductor layers. The semiconductor layers include an n-type window layer and a p-type absorber layer providing a p-n junction, near which photo-conversion occurs. During operation, photons pass through the window layer and are absorbed by the absorber layer. This produces photo-generated electron-hole pairs, the movement of which, promoted by a built-in electric field, produces electric current that can be output to other electrical devices.
A thin film PV device typically has an optically transparent substrate. This substrate can be any suitable, transparent substrate material. Suitable materials include glass, such as soda-lime glass or float glass, etc., and polymer (sheet or plates). A first of two conductive electrodes is provided over the transparent substrate. This first conductive electrode can, in some instances, be just a transparent conductive oxide (TCO) layer (e.g., indium tin oxide, cadmium stannate, SnO2:F, or other transparent, conductive materials). In other instances, this first conductive electrode can be a TCO layer that is associated with a barrier layer (e.g., SiO2, SnO2 or a layered sequence of the two) between it and the transparent substrate for preventing diffusion of sodium from the substrate into other layers of the device, and an optional buffer layer (e.g., a metal oxide such as SnO2, ZnO, or ZnO:SnO2) over the TCO layer for providing a smooth surface upon which subsequently formed semiconductor layers may be deposited. The barrier, TCO and buffer layers are often referred to as a TCO stack since they may first be formed and then deposited unto the substrate as a stack.
The semiconductor layers can be a bi-layer that includes the n-type semiconductor window layer and the p-type semiconductor layer. The n-type semiconductor layer can be made of various semiconductor materials including, but not limited to, cadmium sulfide (CdS). The p-type semiconductor absorber layer can also be made of various semiconductor materials, including, but not limited to, cadmium telluride (CdTe). In some devices, the window layer is desired to be as thin as possible so as to allow the maximum amount of light to reach the absorber layer, but still be sufficiently thick so as to maintain a consistent junction with the absorber layer. Over the semiconductor bi-layer, the second of the two conductive electrodes may be provided. This second electrode is usually referred to as a back contact layer, which is generally made of a metal or alloy (e.g., Mo, Al, Cu, Ag, Au, or combinations of these).
A back cover can be provided over the back contact layer to provide, together with the substrate, support for the PV device. An interlayer (e.g., a polymer) can be provided between the back contact layer and the back cover and over the sides of the other layers of the PV device to seal the PV device from the environment. Such a PV device can be fabricated beginning with the substrate and subsequently depositing or providing the other layers in sequence, or it can be fabricated beginning with the back cover and proceeding with depositing or providing the other layers in the reverse order.
During the manufacture of conventional PV devices having a CdTe-based absorber layer and a CdS-based window layer, a chloride activation process is typically employed to improve efficiency and to reduce electrical anomalies. Such an activation process provides grain growth and repairs (or passivates) defects in the CdTe absorber layer by incorporation of Cl atoms (or ions) into the absorber layer. As discussed below, grain growth and defect repair improves device efficiency by increasing photocurrent and open-circuit voltage (Voc—one of the factors contributing to PV device efficiency and a measure of the maximum voltage the device can produce) and reducing shunting (i.e., unwanted electrically conductive regions in the absorber layer material due to compositional inconsistencies). Efficiency, in this instance, refers to the electrical power (energy) generated by the PV device compared to the equivalent energy of photons incident on the device.
Typically, the activation process includes a first step in which chlorine is introduced to the semiconductor layers, and a second step in which the semiconductor layers are annealed at an elevated temperature for a particular length of time. To introduce the chlorine to the semiconductor layers, CdCl2, for example, may be applied as an aqueous solution (CdCl2 is soluble in water) at a concentration of about 100-300 g/L. It is also possible to use other chlorine-doping materials as alternatives to CdCl2, such as MnCl2, ZnCl2, NHCl4, TeCl2 and MgCl2, for example. For example, the annealing temperature can be about 350°-450° C. and applied for about 60 minutes, with a soaking time of about 15 minutes. Soaking time refers to the time period where the annealing step plateaus at a maximum desired temperature.
Grain growth of the CdTe material occurs as the activation step enlarges the grains, or crystallites, of the CdTe material of the absorber layer. Typically after the CdTe material for the absorber layer is deposited over the CdS material of the window layer, the CdTe material is composed of separate crystallites of CdTe smaller than a micron in size. The activation step promotes recrystallization and grain growth of these crystals, which changes the morphology of the absorber layer. The recrystallization of the CdTe material can take two forms: (1) intragrain, or primary, recrystallization that changes grain orientation and (2) intergrain, or secondary, recrystallization resulting from grain coalescence. This recrystallization, particularly the intergrain type, results in grain growth and larger crystallites of CdTe. Both forms of recrystallization reduce the resistivity of the CdTe material and, by creating acceptor states caused by the incorporation of Cl, make the absorber layer material more p-type, which improves the p-n junction for photoconversion.
As mentioned above, the activation step can provide defect repair (passivation) of the absorber layer, which refers to mitigating photocurrent loss due to, for example, chemical impurities, vacancies, and chemical substitutions, particularly at the grain boundaries in the absorber layer material. Imperfections or defects disrupt the periodic structure in the absorber layer and can create areas of high resistance or current loss. During the CdCl2 anneal of the activation step, the CdS material of the window layer tends to dissolve into and intermix with CdTe of the absorber layer, which makes the CdS window layer have a non-uniform thickness or in some cases it may become discontinuous. This can cause device performance degradation. It would be desirable to use more chlorine-containing dopant, higher annealing temperatures, and/or longer anneal duration in an activation step to more aggressively treat the absorber layer, as this would increase the benefits conferred on the absorber layer by the activation step. However, using more aggressive process conditions during the activation step can cause further dissolving/intermixing of the CdS material (e.g., increased CdS/CdTe intermixing), thus further degrading or destroying the window layer, which causes more degradation in device performance.
An apparent solution to this problem would appear to be to simply increase the initial thickness of the CdS window layer so that if, and when, some of the CdS material is dissolved during the activation step, enough CdS material remains to maintain a good junction. This apparent remedy, however, causes other problems. CdS is relatively light absorbent and having a thicker CdS window layer after the activation step reduces the available light for photon harvesting at the absorber layer, thereby reducing photovoltaic efficiency. In general, it is desired to have a very thin CdS window layer to provide better light transmission to the absorber layer.
A PV device incorporating an absorber layer that can be activated with an aggressive activation step while maintaining the integrity of a thin CdS window layer is desired, as is a method of making such a PV device.